1. Field of the Invention
The present invention relates to a process for modifying fats and oils with an enzyme, and more specifically relates to a process for modifying fats and oils which comprises applying partial glyceride lipase (lipase which does not act on triglycerides but acts on diglycerides and/or monoglycerides is referred to as partial glyceride lipase in the present application) to fats and oils containing partial glycerides (whereby diglycerides and monoglycerides are referred to) to synthesize triglycerides from the included partial glycerides by esterification without changing the fatty acid composition of triglycerides (hereinafter referred to as triglyceride composition) in the fats and oils.
Namely, the present invention aims to obtain in a high yield fats and oils containing triglycerides in high content by esterifying partial glycerides which are impurities contained in the fats and oils without change in the triglyceride composition.
2. Description of the Prior Art
Fats and oils contain triglycerides as main components, and monoglycerides and diglycerides, which are partial glycerides, free fatty acids, etc. as additional components.
The above partial glycerides and free fatty acids are those formed by enzymatical or non-enzymatical hydrolysis during storage, oil extraction and/or purification. The free acids and monoglycerides can be removed by known methods such as alkali deacidification and steam distillation, but no effective industrial separating method for diglycerides has been found.
Recently, a method has been reported wherein oils of high triglycerides content are obtained by synthesizing triglycerides from diglycerides by esterification using a triglyceride lipase. However, transesterification progresses together with ester synthesis in these reaction, and it is intrinsically impossible to avoid change in triglyceride composition.
Further, a method has been reported as a method for removal of diglycerides using an enzyme wherein only diglycerides are selectively hydrolyzed with partial glyceride lipase (Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No. 62-287). This method is excellent in that the enzyme has almost no reactivity with triglycerides and thus high triglyceride content oil can be obtained.
However, some difficulties follows when this method is industrially practiced. First of all, yield is lowered as a natural result of carrying out hydrolysis. Further, it is necessary to separate glycerol which is a hydrolyzate, from the reaction solution. Further, as for the fatty acids which are hydrolyzates, their concentration are high and thus purification yield is extremely lowered under adoption of conventional alkali deacidification. Therefore, it is compelled to utilize physical deacidification, namely steam distillation, which, however, causes a problem that hydrolysis autocatalytically progresses owing to high temperature treatment under high fatty acid concentration to result in lowering of yield and formation of diglycerides. For these problems, it is still difficult to industrially practice this method, and a better method is now desired.
Diglycerides whose separation is difficult as above mentioned cause problems, for example, that (1) they interfere with the formation of crystal nuclei of triglycerides and lower crystallization rate of triglycerides, (2) they inhibit transition rate into stable crystal shape, namely crystal transition, of triglycerides, (3) they makes fractionation of triglycerides difficult based on action of lowering SFC (solid fat content) and (4) they promote rise of acid value when used as fry oil.
A method for solving these problems wherein triglycerides are synthesized from diglycerides by esterification using conventional triglyceride lipase changes triglyceride composition by transesterification which simultaneously takes place and as a result gives an oil having changed various physical properties, and therefore cannot be applied when change in triglyceride composition would be avoided.
Further, a method of removing diglyceride by hydrolysis using partial glyceride lipase has a problem in purification and yield.